1. Field of the Invention
The present invention generally relates to a measuring method and system, especially a method for measuring a CNC machine tool and the system thereof.
2. Description of the Prior Art
There are two types of errors in a contour-following motion of a multi-axis feed drive system: (1) tracking error; and (2) contour error. As shown in FIG. 1, Pd is the current theoretical position and Pa is the actual position with respect to Pd. A tracking error (i.e. a positional error) is a difference between an input position and an actual position in single-axial motion. A contour error is an error ε between a motion path and an originally designed path due to disharmony in multi-axis motion; in other words, it is the minimum distance from the response point of the actual position to the desired path. Thus, a contour error is determined by creating a motion from the response point of the actual position towards the designed trajectory in the normal direction.
In terms of applications, the multi-axis contour following motion may include point-to-point drilling or boring. In these cases, machining precision of the position error is the main concern while the effect of the contour error on machining precision is relatively indifferent. On the other hand, applications such as milling and turning, machining precision is closely related to the trajectory during machining, that is, the so-called contour error is far more important than the single-axis tracking error. Here, a displacement for each axis is created by a simultaneous moving path. When the position error for each axis causes the contour error of the simultaneous moving path to occur, the contour error is compensated using a cross-coupled controller, which generates an adjustment value for each axis and feedback the adjustment values to respective axes, such that each axis has a new displacement. This process is repeated until the required contour precision is achieved.
A motion error created by a platform of a CNC machine tool can be measured by instruments, for example, linear tools, laser interferometers and multifaceted polygonal mirrors. U.S. Pat. No. 6,049,377 discloses a six-axis laser measuring system that can measure six types of positioning errors of a linearly moving platform all at once.
U.S. Pat. No. 4,435,905 discloses a double ball bar (DBB) measuring tool for measuring both static and dynamic errors when a platform is driven to move in a circular path by a CNC machine tool. This measuring tool comprises a extension rod and a pair of gage balls at either end of the extension rod. The displacement change between the two gage balls can be measured by a linear variable differential transformer (LVDT). During measurement, the two gage balls are driven in circular motion, wherein the change in distance between the two gage balls reflects the static and dynamic errors when the platform is driven in circular motion. The measuring results of the DBB measuring tool helps determining static and dynamic motion errors of the linear axis of the CNC machine tool. Errors such as pitch error, squareness error, lost-motion error, clearance error, driver gain error and stick-slip error can be measured. This DBB measuring tool is now only applicable in measuring static and dynamic errors of two linear axes.
U.S. Pat. No. 5,428,446 describes a laser ball bar (LBB) measuring tool comprising a laser measuring device on an extension rod. Thus, the range of measurement for LBB is wider than a LVDT DBB measuring tool.
The abovementioned measuring systems for single- or multi-axis feed machine tools are usually expensive and difficult to operate. Machine tool manufactures need to spend a considerable amount of money to buy these expensive measuring systems, thus raising the cost of the machine tools.